Muscarinic receptors occur on autonomic effector cells and nerve endings, where they mediate effector responses and modulate transmitter release.
Pharmacology and location
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Muscarinic receptors are selectively stimulated by muscarine and blocked by atropine.
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They are located primarily on autonomic effector cells in the heart, blood vessels, eye, smooth muscles and glands of the gastrointestinal, respiratory and urinary tracts, and sweat glands.
Modulation of transmission
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Autonomic ganglia primarily function through nicotinic receptors, but subsidiary muscarinic receptors appear to modulate transmission by inducing a long-lasting late excitatory postsynaptic potential.
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Muscarinic autoreceptors are present prejunctionally on postganglionic cholinergic nerve endings. Their activation inhibits further acetylcholine release.
Subtype organization
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Muscarinic receptors are divided into five subtypes. The first three are the major subtypes and occur on both effector cells and prejunctional nerve endings.
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Muscarinic agonists show little subtype selectivity, whereas some relatively selective antagonists have been produced.